Add generate() functionality to TF 2.0 (#3063)

* add first copy past test to tf 2 generate

* add tf top_k_top_p_filter fn

* add generate function for TF

* add generate function for TF

* implemented generate for all models expect transfoXL

* implemented generate for all models expect transfoXL

* implemented generate for all models expect transfoXL

* make style

* change permission of test file to correct ones

* delete ipdb

* delete ipdb

* fix bug and finish simple gpt2 integration test

* clean test file

* clean test file

* make style

* make style

* make style

* make style

* change import style

* change import style

* make style

* make style

* add decorators

* add decorators

* fix tf ctrl bug dim => axis in TF

* make style

* make style

* refactored test file

* refactored test file

* take out test_torch_tf_conversion if nothing is defined

* take out test_torch_tf_conversion if nothing is defined

* remove useless files

* remove useless files

* fix conflicts

* fix conflicts

* fix conflicts

* fix conflicts

* fix conflicts

* solve conflicts

* solve conflicts

* fix conflicts

* fix conflicts

* merge conflicts

* delete ipdb

* exposed top_k_top_p_filtering fns

* delete weirdly created w! file

* add comment to test tf common modeling

* fix conflicts

* fix conflicts

* make style

* merge conflicts

* make style

* change tf.tensor.shape to shape_list(tensor)

src/transformers/modeling_ctrl.py

@@ -454,14 +454,12 @@ class CTRLLMHeadModel(CTRLPreTrainedModel):
     def get_output_embeddings(self):
         return self.lm_head
 
-    def prepare_inputs_for_generation(self, input_ids, **kwargs):
+    def prepare_inputs_for_generation(self, input_ids, past, **kwargs):
         # only last token for inputs_ids if past is defined in kwargs
-        if "past" in kwargs and kwargs["past"]:
+        if past:
             input_ids = input_ids[:, -1].unsqueeze(-1)
 
-        inputs = {"input_ids": input_ids}
-        inputs.update(kwargs)
-        return inputs
+        return {"input_ids": input_ids, "past": past}
 
     @add_start_docstrings_to_callable(CTRL_INPUTS_DOCSTRING)
     def forward(